A ferroelectric film of PZT or the like formed by using a chemical solution deposition (CSD) method represented by a sol-gel method cannot be used as a piezoelectric material immediately after being formed, and a polarization process is necessarily performed in order to use the ferroelectric film in a gyro sensor or the like. In a case of using this film in a sensor such as a pyroelectric sensor or a gyro sensor, a performance index g of a piezoelectric film used is represented by the following Formula (1).g(V·m/N)=d31/∈33  (1)
In Formula (1), d31 represents a piezoelectric constant and ∈33 represents a permittivity.
That is, in a case of using a ferroelectric film of PZT or the like in a sensor such as a pyroelectric sensor or a gyro sensor, it is generally desirable that a piezoelectric constant of a film be great and a permittivity or a dielectric loss (tan δ) of a film be low. In addition, it is desirable that polarization directions of a film be aligned immediately after a film is formed from the viewpoints of stability of polarization and omitting a polarization step.
In a case of using such a film in an actuator of an ink jet head or the like, the film is used by applying a high voltage, and accordingly, a polarization process is not necessary. This is because, in a case of using a ferroelectric film by applying a high voltage, polarization is performed with a driving voltage, even when polarization directions of the film are not aligned immediately after the film is formed, for example. However, even if a polarization process is performed, depolarization may occur at the time of a thermal treatment such as a reflowing process after the polarization process.
Regarding such problems, a self-poling phenomenon of a PZT film has been investigated and the result thereof shows possibilities that pinning of domains due to defects in a film is a main reason for the phenomenon (for example, see Non-Patent Document 1).
Also, in a field of piezoelectric bodies, there is a problem regarding productivity when performing film formation by using a CSD method. In a case of considering productivity, it is desired that a deposition rate be as high as possible, but, in a case of forming a PZT film by using a sol-gel method, for example, a high-temperature process such as calcination or sintering is generally performed in a sol-gel method. Thus, when it is desired to obtain a thicker film by increasing a single coating amount, tensile stress generated in a film at the time of sintering or the like increases and cracks may be generated on the formed film. When cracks are generated on the formed film, electric properties of a ferroelectric film are deteriorated. Accordingly, in the related art, a thickness of a film to be formed by a single application is limited to be approximately 100 nm in a sol-gel method, and in a case of forming a ferroelectric film having a certain thickness, a method of repeating application or sintering of a composition several times has been employed.
However, in this method, production efficiency is deteriorated and manufacturing cost is increased. Therefore, improvement related to materials, that is, research or development of a raw material solution which causes an increase in a film thickness formed by a single application, without generating cracks, has been performed. Patent Document 1, for example, discloses a raw material solution for forming a metal oxide thin film containing Ti in which propylene glycol is added to the raw material solution. A film having a thickness equal to or greater than 0.2 μm can be formed by a single application of this raw material solution, without generating cracks.
In addition, a CSD method represented by a sol-gel method and a sputtering method are well known as a method of manufacturing a PZT-based film used as a piezoelectric micro-electro mechanical system (MEMS) device. A PZT-based film manufactured by a CSD method generally has a high dielectric breakdown voltage and therefore is suitable as a high-voltage driving device such as an ink jet head. In contrast, a PZT-based film manufactured by a sputtering method has compressive stress due to an injection effect at the time of forming a film in many cases, and is oriented to the (001) plane. Therefore, a film excellent in low-voltage driving can be formed. It is also possible to realize a self-poling phenomenon of aligning polarization directions immediately after forming a film by designing a film-forming method.
In a case where a ferroelectric thin film such as a PZT-based film is manufactured as a sensor element of a gyro sensor by using a sputtering method, although there is concern that a polarization state may be damaged due to a thermal treatment for soldering in a reflowing process after packaging the film, there is an internal bias in a film realizing a self-poling phenomenon, and therefore, it is advantageous that a polarization state not be damaged due to this thermal treatment.
In a case where a ferroelectric thin film such as a PZT-based film is manufactured by using a CSD method, the manufactured ferroelectric thin film has excellent reproducibility of properties or property uniformity in a wafer surface, from the viewpoints of properties of the formed film. In addition, since a vacuum state is not used in the CSD method, the equipment cost is significantly decreased, compared to that of the sputtering method. In order to realize such advantages, a PZT-based film having excellent temperature characteristics in the CSD without damaging a polarization state is obtained.
As described above, research regarding a self-poling phenomenon of a PZT-based film has been done and Non-Patent Document 1 discloses that a self-poling phenomenon is also realized in a PZT-based film in the CSD method. Non-Patent Document 1 discloses that a self-poling phenomenon is caused by a strain of a film due to an effect of strain due to inconsistency of lattices in substrate interfaces at a thin film level.
In addition, Patent Document 2 discloses a manufacturing method of a ferroelectric thin film of forming an orientation control layer in which crystal orientation is controlled to the (100) plane on a lower electrode which will be described later. Further, Patent Document 3 discloses a manufacturing method of a ferroelectric thin film of forming an orientation control layer in which crystal orientation is controlled to the (110) plane on a lower electrode which will be described later.